1. Field of the Invention
The present invention relates to a semiconductor device including an analog circuit including a MOS transistor.
2. Description of the Related Art
Description is given of a semiconductor device including an analog circuit including a MOS transistor (hereinafter referred to as “MOS analog circuit”) according to the related art.
As an example of the related-art MOS analog circuit, a current source circuit used in a digital-to-analog converter is illustrated in FIG. 5.
The current source circuit of FIG. 5 includes a current source 501, current mirror circuits 502 and 503, a load resistor 504, a switching circuit 505, a terminal 506, a power supply terminal 507, and a ground terminal 508.
The current mirror circuit 502 includes nMOS transistors 511 and 512, and the current mirror circuit 503 includes pMOS transistors 513 and 514. The switching circuit 505 is formed of a pMOS transistor, and opening and short-circuiting of the switching circuit 505 is controlled through a digital signal supplied by the terminal 506.
A current output by the current source circuit 501 is input to an input terminal 515 of the current mirror circuit 502, to thereby be output from an output terminal 516.
When the switching circuit 505 is in an OFF state, the current mirror circuit 503 is operated. Therefore, a current output from the output terminal 516 of the current mirror circuit 502 is input to an input terminal 517 of the current mirror circuit 503, and the mirrored current is output from an output terminal 518 thereof. The current output from the output terminal 518 of the current mirror circuit flows through the load resistor 504, and generates an analog signal voltage across both ends of the resistor 504.
Further, when the switching circuit 505 is in an ON state, the current mirror circuit 503 is stopped. That is, the pMOS transistors 513 and 514 are placed in an OFF state because a gate-source voltage Vgs becomes 0 V. Then, the current output from the output terminal 518 of the current mirror circuit 503 becomes 0. Therefore, the voltage generated across both ends of the load resistor 504 becomes 0.
As described above, the current source circuit of FIG. 5 is configured to generate an analog voltage, which corresponds to a digital signal supplied by the terminal 506, across both ends of the load resistor 504 (see, for example, Japanese Patent Application Laid-open No. Hei 11-251912).
Meanwhile, in recent years, industrial machines and in-vehicle devices have become increasingly computerized, and semiconductor components are being mounted on various electronic devices. For example, semiconductor devices using the MOS transistor tend to be increasingly used also inside automobiles in which high reliability is required under strict operating conditions.
In general, measures for achieving high reliability in the semiconductor device include removal of potentially defective products at a burn-in step before shipping. In the burn-in step, revealing failure is accelerated by operating the products under high-temperature and high-voltage conditions for a long period of time. However, there is a problem in that a manufacturing cost is increased and an accuracy is decreased due to the fact that, in order to actually perform a burn-in test, there needs to be prepared a testing environment in which high temperature and high voltage can be applied to the products.
Specifically, in the related-art MOS analog circuit, when the circuit is in an operating state, each MOS transistor inside the circuit is biased by a constant current. Thus, the gate-source voltage Vgs is low, being near Vth. Therefore, a high electrical field cannot be applied to a gate oxide film. As a result, when the gate oxide film includes latent defects that have a major impact on the reliability of the MOS transistor, a measure of placing the circuit under a high-temperature environment for a long period of time needs to be taken because the revealing the failure cannot be accelerated by the electrical field in the burn-in step. Consequently, there has been a problem in that a test facility is needed in order to take those measures, which leads to an increase in the cost of the burn-in test for the MOS analog circuit to be mounted on the electronic devices for which high reliability is required.
Further, in the MOS analog circuit, the current of an operating circuit flows based on a current value of a constant current source. Therefore, there has been a problem in that, even when a leakage current is generated due to the defects in a gate of the MOS transistor of the MOS analog circuit, there is a low possibility that the leakage current can be detected by measuring a current between the power supply terminal and the ground terminal.